Mutation rates are expressed in mutations per bp, not mutations per SNP.

There are 25 million (useful) bp on the Y chromosome, not 50,000. If
you want to know how many SNPs per birth you are likely to observe,
mutliply the mutation rate (which is closer to 1-in-30 million bps,
actually, for the Y) by the number of observed sites.

(1/30,000,000) * 25,000,000 = .83

That is, we should observe a Y-SNP difference every birth or two.

With billions of men on the planet, and only 25 million possible Y-SNP
locations, it must follow that every SNP is happening multiple times
per generation.

However, this is not cause for concern because - as Diana pointed out
- phylogeneticists always use the SNPs in the context of other SNPs.
If someone in J2 turns up L21+, that won't confuse us because it will
be obvious from all his other SNPs (M429,M304,M172, etc.) that he is
actually J2.

VV

On Mar 15, 2010, at 9:06 AM, Steven Bird wrote:

> If we have an average of about 50000 SNPs on the Y chromosome
> (that's 51 million BP divided by an average of 1 SNP every 1000 bp -
> I've rounded a bit for convenience) then the odds of any one SNP
> changing on any one site is .00005 per transmission, given a
> mutation rate of 1 mutation in 1 billion transmissions